1. Field of the Invention
The invention generally relates to marking instruments, and, more specifically, to free ink marking instruments that provide greater hydrostatic stability in response to changes in temperature and pressure, improved ink flow performance, improved design freedom, and ease of manufacture.
2. Brief Description of Related Technology
It is well known to provide a pen having free ink (i.e., liquid ink that can be stored in a cavity and that is free to move or flow in response to external forces such as motion, gravity, and pressure) that a user can selectively apply to a substrate such as paper, metal, or plastic. Such known pens typically include a reservoir for storing the ink and a channel for directing the ink from the reservoir to a marking tip. The ink of such known pens typically has a vapor pressure such that the ink, and any air in the reservoir, expands and contracts in response to changes in ambient temperature and pressure. Such expansion and contraction can cause the ink to leak from the writing tip of the pen, under certain conditions.
Other such known pens include a buffer for storing ink that would otherwise leak through the tip in response to changes in ambient temperature and pressure. The excess ink is typically stored in the front of the buffer, near the tip of the pen, due to gravity, when the pen is in the tip-down position. However, such known pens have several disadvantages: the ink capacity of the buffer is limited such that when the buffer is full the excess ink leaks from the pen, and the ink is often permanently stored in the buffer resulting in decreased buffer capacity and wasted ink. Another of such known pens provides for the clearing of ink from the buffer when the pressure inside the pen increases by venting air into the pen through an external vent. Such known pens, however, clear only a small portion of the buffer. Still other pens have achieved hydrostatic stability, but only with design restrictions that require stringent manufacturing tolerances and result in reduced ink flow rates.
Accordingly, it would be desirable to provide a hydrostatically stable pen that responds to repeated temperature and pressure changes without substantially leaking or dripping, and that permits greater design freedom and ink flow rates.
It is an objective of the invention to overcome one or more of the problems described above.
Accordingly, one aspect of the invention is a free ink marking instrument for dispensing an ink, including a housing, a reservoir for storing ink within the housing, a feed tube to convey ink communicating with the reservoir, a tip disposed within the feed tube for conveying ink to a substrate at a marking end of the tip, a porous buffer disposed within the housing adjacent the feed tube and configured for storing ink during periods of a decreasing pressure differential between the reservoir and the atmosphere, and a vent hole in the feed tube, wherein the vent hole is disposed at a distance greater than the length of the tip, measured from the marking end of the tip.
Another aspect of the invention is a free ink marking instrument for dispensing an ink including a housing, a reservoir for storing ink within the housing, a feed tube to convey ink communicating with the reservoir, wherein the feed tube has primary and secondary ends at one extremity, a tip disposed within the feed tube end for conveying ink to a substrate at a marking end of the tip, a porous buffer disposed within the housing adjacent the feed tube and configured for storing ink during periods of a decreasing pressure differential between the reservoir and the atmosphere, and a vent hole formed between a secondary end of the feed tube and a butt end of the tip.
Still another aspect of the invention is a free ink marking instrument for dispensing an ink including a housing, a reservoir for storing ink within the housing, a feed tube to convey ink communicating with the reservoir, a tip disposed within the feed tube for conveying ink to a substrate at a marking end of the tip, a porous buffer disposed within the housing adjacent the feed tube and configured for storing ink during periods of a decreasing pressure differential between the reservoir and the atmosphere, and a passage between the outside surface of the tip and the inside surface of the feed tube, wherein the passage has a mean thickness of about 0.010 inches (in.) to about 0.025 in. (about 0.254 mm to about 0.635 mm).
Further aspects and advantages of the invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the appended claims. While the invention is susceptible of embodiments in various forms, described hereinafter are specific embodiments of the invention with the understanding that the disclosure is illustrative, and is not intended to limit the invention to the specific embodiments described herein.